I have a similar ticking in my '99 C280. It sounds like a bad plug wire arcing against the head 2 to 3 times per second, but I can't see any spark in the dark, so that's not it. It doesn't sound at all internal, nor does it seem to sound off continually.

Can anyone provide more information about the "minor modification to the purge valve line mount?"

Dan

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2003 Hyundai Santa Fe, Silver
1999 C280 Sport, Black
1997 Ford Escort, Toreador Red